Rainfall Conditions in Equatorial East Africa during the Nineteenth Century as Inferred from the Record of Lake Victoria

The East African lakes have exhibited dramaticfluctuations on both historical and paleo-climatictime scales. Levels of these lakes, and otherhistorical indicators in Africa, suggested thatenvironmental conditions in the nineteenth centurywere much more extreme than anything evident in themodern record. In this study, a water balance modelis used to estimate the rainfall associated with theseconditions, based on the Lake Victoria record. Theresults suggest that the conditions were not unlikeanomalous periods found during the twentieth century,but they may have persisted for somewhat longerperiods of time. The paper also demonstrates ageneric tool that can be applied to interpretinghistorical and paleo-lake levels in quantitative termsof rainfall.     

Reconstruction of integrated temperature series of the past 2,000 years on the Tibetan plateau with 10-year intervals

Using 1,981 pieces of temperature records extracted from a selection of tree rings, ice cores, sediments, and other materials with high-resolution historical temperature proxy data, a temperature series of the past 2,000 years on the Tibetan Plateau (TP) with 10-year intervals was reconstructed by the method of single sample correction—multi-sample average integration equations. This series shows that the warm periods mainly appeared before 235 A.D., 775–1275 A.D. and 1845–2000 A.D., while the cold periods occurred 245–765 A.D., 1045–1145 A.D., and 1285–1835 A.D. The Little Ice Age left clear evidence on the TP and its coldest period was between 1635 and 1675 A.D. The Medieval Warm Period on the TP was not as warm as that in the late twentieth century. During the nineteenth century, overall temperature tends to be warmer with a clear rising trend, and in the late twentieth century new highs broke the record of the past 2,000 years. Power spectrum analysis shows that temperature on the TP changes consistently and evidently in a 150-year cycle. This integrated series also shows clear correlations with sunspot activity and solar radiation, as high sunspot activities generally led to warmer periods, and vice versa. Solar activities and intense radiation of recent years are naturally conducive to the global warming since the nineteenth century. The combination of greenhouse gases and natural fluctuations in climate has been the main culprit behind the global warming in the twentieth century.     

A Semi-Quantitative, Regional Precipitation Data Set for Studying African Climates of the Nineteenth Century, Part I. Overview of the Data Set

This article describes a historical archive of proxy and actual precipitation data that extends the African climate record back to the early nineteenth century. The `proxy' archive includes verbal, documentary references which contain information related to rainfall conditions, such as references to famine, drought, agriculture or the nature of the rainy season. The precipitation archive includes all observations made in Africa during the nineteenth century. It consists of records for 60 stations in Algeria, 87 stations in South Africa and 304 stations scattered over the rest of Africa. Information is particularly plentiful for the 1880s and 1890s. The two parts have been be combined into a semi-quantitative regional data set indicating annual rainfall conditions in terms of anomaly classes (e.g., normal, dry, wet). This data set extends from the early nineteenth century to 1900 and distinguishes seven anomaly classes, using numbers ranging from –3 to +3 to represent very wet, wet, good rains, normal, dry, drought, and severe drought. The regionalization is based on 90 geographical regions shown via studies of the modern precipitation record to be climatically homogeneous with respect to the interannual variability of rainfall. The regional aggregation allows the voluminous fragmentary information available in historical sources to be used systematically to produce multi-year time series that can be directly integrated into the modern record for each region. The resultant time series can also be subjected to statistical analysis, in order to investigate nineteenth century climate over Africa. Spatial detail is added to the data set by utilizing a unique methodology based on climatic teleconnections established from studies of rainfall variability over Africa. The historical information and station records have been combined into a file containing a regional anomaly value for up to 90 geographic regions and the years 1801–1900. Gaps necessarily remain in the matrix, but as early as the 1820s over 40 regions are represented. By the 1880s generally around 70 regions or more are represented.     

Evolution of annual rainfall irregularity in the southwest of the Iberian Peninsula

The areas of the Iberian Peninsula with Mediterranean climate are characterised by rainfall irregularity. Standard statistical estimation methods provide a limited insight of all the dimensions of such irregularity. Based on different techniques to describe the inter-annual irregularity of rainfall, the authors develop a new method: the disparity indices. These indices are then applied to several historical rainfall series (dating from the end of the nineteenth century up to the present) from the southwest of the Iberian Peninsula. Similar rainfall irregularity patterns are found in all weather stations in the studied area, confirming their belonging to the same climatic region. The results indicate a relative stability during the first third of the twentieth century, coinciding with a period of low precipitation and a progressive increase during the last three decades. The use of a new index named specific disparity index has proven be useful in highlighting the irregularity within the rainfall series at each meteorological station. This new index could contribute to monitor future changes in precipitation within the general framework of research on climate change. Although Mediterranean ecosystems are adapted to important fluctuations in the rainfall regime, this increase in irregularity may affect rivers, wetlands and the hygrophytic vegetation.     

‘Splendid rains have fallen’: links between El Niño and rainfall variability in the Kalahari, 1840–1900

This study uses a range of published and unpublished historical documentary sources to explore the nature of rainfall variability in the Kalahari Desert and adjacent hardveld regions of central southern Africa during the seventeen Pacific El Niño–Southern Oscillation (ENSO) episodes that occurred between 1840 and 1900. Documentary data are used in two ways. First, maps of relative annual rainfall levels are presented for each of the 12 single-year and five protracted ENSO episodes during the period, in order to identify the associated inter-annual rainfall variations. These suggest that the relationship between ENSO episodes and rainfall variability identified for the twentieth century, whereby warm events are frequently preceded by wetter conditions during the austral summer prior to the event year and succeeded by drought in the following summer, has broadly held for much of the last 160 years. This is despite the long-term fluctuations in precipitation and temperature which are known to have occurred over this period. Droughts are identified following at least thirteen of the 17 single-year and protracted ENSO episodes. Pre-ENSO wetter periods are less common, with only nine of the ENSO episodes preceded by above-normal rainfall. Second, the documentary data are analyzed in detail in order to reveal any evidence for high resolution intra-annual variations in the seasonal distribution of rainfall during ENSO events. Seasonal sequences of rainfall/drought appear to have closely followed contemporary patterns, with heavy rainfall commonly occurring late in the pre-ENSO year or early in the ENSO year(s), and drought at the start of the post-ENSO year. This relationship can be seen to hold most strongly for single-year ENSO warm events and for the first year of protracted events, but rainfall conditions were more variable during the later years of protracted events.     

Decadal variability of rainfall in the Sahel: results from the coupled GENESIS-IBIS atmosphere-biosphere model

In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the twentieth century is used as the primary model forcing. Using this coupled global model, we experiment on treating vegetation as a static boundary condition and as a dynamic component of the Earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics act as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model. Regardless how vegetation is treated and what model resolution is used, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as that observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the twentieth century drought in the Sahel.     

Reconstruction of historical pressure patterns over Japan using two-point pressure–temperature datasets since the nineteenth century

The temperature and pressure differences between Tokyo and Nagasaki were used to reconstruct past climate conditions. January and July in each available year since the 1820s were classified into several types with characteristic sea level atmospheric pressure patterns. This results in 18 years of pre-1881 data and a continuous series thereafter. The series indicate that the warming after 1900 (after the end of the so-called Little Ice Age) and again after 1960 can at least partly be attributed to an increase in the frequency of warm circulation pattern types at the expense of cold types. The difference in nature of the shifts in circulation types that occurred in the late nineteenth century compared with that in the late twentieth centuries suggests that the mechanism behind the warming in the late nineteenth century differs from that in the late twentieth century.     

Climate variability and projected change in the western United States: regional downscaling and drought statistics

Climate change in the twenty-first century, projected by a large ensemble average of global coupled models forced by a mid-range (A1B) radiative forcing scenario, is downscaled to Climate Divisions across the western United States. A simple empirical downscaling technique is employed, involving model-projected linear trends in temperature or precipitation superimposed onto a repetition of observed twentieth century interannual variability. This procedure allows the projected trends to be assessed in terms of historical climate variability. The linear trend assumption provides a very close approximation to the time evolution of the ensemble-average climate change, while the imposition of repeated interannual variability is probably conservative. These assumptions are very transparent, so the scenario is simple to understand and can provide a useful baseline assumption for other scenarios that may incorporate more sophisticated empirical or dynamical downscaling techniques. Projected temperature trends in some areas of the western US extend beyond the twentieth century historical range of variability (HRV) of seasonal averages, especially in summer, whereas precipitation trends are relatively much smaller, remaining within the HRV. Temperature and precipitation scenarios are used to generate Division-scale projections of the monthly palmer drought severity index (PDSI) across the western US through the twenty-first century, using the twentieth century as a baseline. The PDSI is a commonly used metric designed to describe drought in terms of the local surface water balance. Consistent with previous studies, the PDSI trends imply that the higher evaporation rates associated with positive temperature trends exacerbate the severity and extent of drought in the semi-arid West. Comparison of twentieth century historical droughts with projected twenty-first century droughts (based on the prescribed repetition of twentieth century interannual variability) shows that the projected trend toward warmer temperatures inhibits recovery from droughts caused by decade-scale precipitation deficits.     

Natural variability of summer rainfall over China in HadCM3

Summer rainfall over China has shown decadal variability in the past half century, which has resulted in major north–south shifts in rainfall with important implications for flooding and water resource management. This study has demonstrated how multi-century climate model simulations can be used to explore interdecadal natural variability in the climate system in order to address important questions around recent changes in Chinese summer rainfall, and whether or not anthropogenic climate change is playing a role. Using a 1,000-year simulation of HadCM3 with constant pre-industrial external forcing, the dominant modes of total and interdecadal natural variability in Chinese summer rainfall have been analysed. It has been shown that these modes are comparable in magnitude and in temporal and spatial characteristics to those observed in the latter part of the twentieth century. However, despite 1,000 years of model simulation it has not been possible to demonstrate that these modes are related to similar variations in the global circulation and surface temperature forcing occurring during the latter half of the twentieth century. This may be in part due to model biases. Consequently, recent changes in the spatial distribution of Chinese summer rainfall cannot be attributed solely to natural variability, nor has it been possible to eliminate the likelihood that anthropogenic climate change has been the driving factor. It is more likely that both play a role.     

Documentary reconstruction of monsoon rainfall variability over western India, 1781–1860

Investigations into the climatic forcings that affect the long-term variability of the Indian summer monsoon are constrained by a lack of reliable rainfall data prior to the late nineteenth century. Extensive qualitative and quantitative meteorological information for the pre-instrumental period exists within historical documents, although these materials have been largely unexplored. This paper presents the first reconstruction of monsoon variability using documentary sources, focussing on western India for the period 1781–1860. Three separate reconstructions are generated, for (1) Mumbai, (2) Pune and (3) the area of Gujarat bordering the Gulf of Khambat. A composite chronology is then produced from the three reconstructions, termed the Western India Monsoon Rainfall reconstruction (WIMR). The WIMR exhibits four periods of generally deficient monsoon rainfall (1780–1785, 1799–1806, 1830–1838 and 1845–1857) and three of above-normal rainfall (1788–1794, 1813–1828 and 1839–1844). The WIMR shows good correspondence with a dendroclimatic drought reconstruction for Kerala, although agreement with the western Indian portion of the tree-ring derived Monsoon Asia Drought Atlas is less strong. The reconstruction is used to examine the long-term relationship between the El Nino-Southern Oscillation (ENSO) and monsoon rainfall over western India. This exhibits peaks and troughs in correlation over time, suggesting a regular long-term fluctuation. This may be an internal oscillation in the ENSO-monsoon system or may be related to volcanic aerosol forcings. Further reconstructions of monsoon rainfall are necessary to validate this. The study highlights uncertainties in existing published rainfall records for 1817–1846 for western India.     

From headwater tributaries to international river: Observing and adapting to climate variability and change in the Nile basin

Egypt is almost totally dependent upon water that originates from the upstream headwaters of the Nile in the humid Ethiopian and East African highlands. Analysis of rainfall and river flow records during the 20th century demonstrates high levels of interannual and interdecadal variability. This is experienced locally and regionally in the headwater regions of the Nile and internationally through its effects on downstream Nile flows in Sudan and Egypt. Examples of climate variability are presented from areas in the basin where it exerts a strong influence on society; the Ethiopian highlands (links with food security), Lake Victoria (management of non-stationary lake levels) and Egypt (exposure to interdecadal variability of Nile flows). These examples reveal adaptations across various scales by individuals and institutions acting alongside other social and economic considerations.Water resources management in the downstream riparian Egypt has involved institutional level reactive adaptations to prolonged periods of low and high Nile flows. Observed responses include the establishment of more robust contingency planning and early warning systems alongside strategic assessment of water use and planning in response to low flows during the 1980s. In the 1990s high flows have enabled Egypt to pursue opportunistic policies to expand irrigation. These policies are embedded in wider socio-political and economic considerations but increase Egypt's exposure and sensitivity to climate driven fluctuations in Nile flows. Analysis of climate change projections for the region shows there is no clear indication of how Nile flows will be affected because of uncertainty about future rainfall patterns in the basin. In many instances the most appropriate entry point for adaptation to climate change will be coping with climate variability and will play out against the certainty of looming national water scarcity in Egypt due to rapid population growth and its possible exacerbation by water demands from upstream riparians.     

The response of the South Asian Summer Monsoon circulation to intensified irrigation in global climate model simulations

Agricultural intensification in South Asia has resulted in the expansion and intensification of surface irrigation over the twentieth century. The resulting changes to the surface energy balance could affect the temperature contrasts between the South Asian land surface and the equatorial Indian Ocean, potentially altering the South Asian Summer Monsoon (SASM) circulation. Prior studies have noted apparent declines in the monsoon intensity over the twentieth century and have focused on how altered surface energy balances impact the SASM rainfall distribution. Here, we use the coupled Goddard Institute for Space Studies ModelE-R general circulation model to investigate the impact of intensifying irrigation on the large-scale SASM circulation over the twentieth century, including how the effect of irrigation compares to the impact of increasing greenhouse gas (GHG) forcing. We force our simulations with time-varying, historical estimates of irrigation, both alone and with twentieth century GHGs and other forcings. In the irrigation only experiment, irrigation rates correlate strongly with lower and upper level temperature contrasts between the Indian sub-continent and the Indian Ocean (Pearson’s r = ?0.66 and r = ?0.46, respectively), important quantities that control the strength of the SASM circulation. When GHG forcing is included, these correlations strengthen: r = ?0.72 and r = ?0.47 for lower and upper level temperature contrasts, respectively. Under irrigated conditions, the mean SASM intensity in the model decreases only slightly and insignificantly. However, in the simulation with irrigation and GHG forcing, inter-annual variability of the SASM circulation decreases by ~40 %, consistent with trends in the reanalysis products. This suggests that the inclusion of irrigation may be necessary to accurately simulate the historical trends and variability of the SASM system over the last 50 years. These findings suggest that intensifying irrigation, in concert with increased GHG forcing, is capable of reducing the variability of the simulated SASM circulation and altering the regional moisture transport by limiting the surface warming and reducing land–sea temperature gradients.     

Characteristics of anomalous precipitation events over eastern China during the past five centuries

Characteristics of anomalous precipitation events during the past five centuries in North China (NC) and the middle-lower Yangtze River Valley (MLYRV) were investigated using the data network of dryness/wetness index (DWI) over eastern China. The high occurrence frequency of anomalous precipitation events mainly occurred at periods of high solar forcing, active volcanic eruption, and large anthropogenic forcing (the twentieth century). Coherence and dipole were the two dominant modes in spatial patterns of anomalous precipitation events. Coherent floods dominated the eighteenth and nineteenth centuries, whereas coherent droughts occurred frequently in the seventeenth and twentieth centuries. The dipole patterns of anomalous precipitation events were the most frequent in the twentieth century. NC experienced more floods in the cold periods than warm periods. Both NC and the MLYRV experienced far fewer droughts and more floods in the warm eighteenth century when natural climate forcing dominated, and more droughts in the twentieth century when anthropogenic forcing dominated. Coherent drought was the only spatial pattern of precipitation significantly associated with explosive low-latitude volcanic eruptions. The increased coherent droughts and dipole patterns in the twentieth century support the findings of previous modeling studies that the tropospheric aerosols and human-induced land cover changes play important roles in the changes of summer rainfall over eastern China. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. This paper is a contribution to the AMIP-CMIP Diagnostic Sub-project on General Circulation Model Simulation of the East Asian Climate, coordinated by W.-C. Wang.     

African monsoon teleconnections with tropical SSTs: validation and evolution in a set of IPCC4 simulations

A set of 12 state-of-the-art coupled ocean-atmosphere general circulation models (OAGCMs) is explored to assess their ability to simulate the main teleconnections between the West African monsoon (WAM) and the tropical sea surface temperatures (SSTs) at the interannual to multi-decadal time scales. Such teleconnections are indeed responsible for the main modes of precipitation variability observed over West Africa and represent an interesting benchmark for the models that have contributed to the fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC4). The evaluation is based on a maximum covariance analysis (MCA) applied on tropical SSTs and WAM rainfall. To distinguish between interannual and multi-decadal variability, all datasets are partitioned into low-frequency (LF) and high-frequency (HF) components prior to analysis. First applied to HF observations, the MCA reveals two major teleconnections. The first mode highlights the strong influence of the El Niño Southern Oscillation (ENSO). The second mode reveals a relationship between the SST in the Gulf of Guinea and the northward migration of the monsoon rainbelt over the West African continent. When applied to HF outputs of the twentieth century IPCC4 simulations, the MCA provides heterogeneous results. Most simulations show a single dominant Pacific teleconnection, which is, however, of the wrong sign for half of the models. Only one model shows a significant second mode, emphasizing the OAGCMs’ difficulty in simulating the response of the African rainbelt to Atlantic SST anomalies that are not synchronous with Pacific anomalies. The LF modulation of these HF teleconnections is then explored through running correlations between expansion coefficients (ECs) for SSTs and precipitation. The observed time series indicate that both Pacific and Atlantic teleconnections get stronger during the twentieth century. The IPCC4 simulations of the twentieth and twenty-first centuries do not show any significant change in the pattern of the teleconnections, but the dominant ENSO teleconnection also exhibits a significant strengthening, thereby suggesting that the observed trend could be partly a response to the anthropogenic forcing. Finally, the MCA is also applied to the LF data. The first observed mode reveals a well-known inter-hemispheric SST pattern that is strongly related to the multi-decadal variability of the WAM rainfall dominated by the severe drying trend from the 1950s to the 1980s. Whereas recent studies suggest that this drying could be partly caused by anthropogenic forcings, only 5 among the 12 IPCC4 models capture some features of this LF coupled mode. This result suggests the need for a more detailed validation of the WAM variability, including a dynamical interpretation of the SST–rainfall relationships.     

A precipitation-dominated,mid-latitude glacier system: Mount Shasta,California

Temperature is often seen as the dominant control on inter-decadal glacier volume changes. However, despite regional warming over the past half-century, the glaciers of Mount Shasta have continued to expand following a contraction during a prolonged drought in the early twentieth century, indicating a greater sensitivity to precipitation than temperature. We use the 110 year record of fluctuations in Mount Shasta’s glaciers and climate to calibrate numerical glacier models of the two largest glaciers. The reconstructed balance and volume histories show a much greater correlation to precipitation than temperature and significant correlation to oscillatory modes of Pacific Ocean climate. An approximately 20% increase in precipitation is needed for every 1°C increase in temperature to maintain stability. Under continued historical trends, oscillations in climate modes and random variability will dominate inter-decadal variability in ice volume. Under the strong warming trend predicted by a regional climate model, the temperature trend will be the dominant forcing resulting in near total loss of Mount Shasta’s glaciers by the end of the twenty-first century.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Global and hemispheric temperature reconstruction from glacier length fluctuations

Temperature reconstructions for recent centuries provide a historical context for the warming over the twentieth century. We reconstruct annual averaged surface temperatures of the past 400?years on hemispherical and global scale from glacier length fluctuations. We use the glacier length records of 308 glaciers. The reconstruction is a temperature proxy with decadal resolution that is completely independent of other temperature records. Temperatures are derived from glacier length changes using a linear response equation and an analytical glacier model that is calibrated on numerical model results. The global and hemispherical temperatures reconstructed from glacier length fluctuations are in good agreement with the instrumental record of the last century. Furthermore our results agree with existing multi-proxy reconstructions of temperature in the pre-instrumental period. The temperature record obtained from glacier fluctuations confirms the pronounced warming of the twentieth century, giving a global cumulative warming of 0.94?±?0.31?K over the period 1830–2000 and a cumulative warming of 0.84?±?0.35?K over the period 1600–2000.     

European storminess: late nineteenth century to present

Annual and seasonal statistics of local air pressure characteristics have already been used as proxies for storminess across Northern Europe. We present an update of such proxies for Northern Europe and an unprecedented analysis for Central Europe which together considerably extends the current knowledge of European storminess. Calculations are completed for three sets of stations, located in North-Western, Northern and Central Europe. Results derived from spatial differences (geostrophic winds) and single station pressure changes per 24 h support each other. Geostrophic winds’ high percentiles (95th, 99th) were relatively high during the late nineteenth and the early twentieth century; after that they leveled off somewhat, to get larger again in the late twentieth century. The decrease happens suddenly in Central Europe and over several decades in Northern Europe. The subsequent rise is most pronounced in North-Western Europe, while slow and steady in Central Europe. Europe’s storm climate has undergone significant changes throughout the past 130 years and comprises significant variations on a quasi-decadal timescale. Most recent years feature average or calm conditions, supporting claims raised in earlier studies with new evidence. Aside from some dissimilarity, a general agreement between the investigated regions appears to be the most prominent feature. The capability of the NAO index to explain storminess across Europe varies in space and with the considered period.     

An intercomparison of observed and simulated extreme rainfall and temperature events during the last half of the twentieth century: part 2: historical trends

We analyze historical simulations of variability in temperature and rainfall extremes in the twentieth century, as derived from various global models run informing the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC-AR4). On the basis of three indices of climate extremes, we compare observed and modeled trends in time and space, including the direction and significance of the changes at the scale of South America south of 10° S. The climate extremes described warm nights, heavy rainfall amounts and dry spells. The reliability of the GCM simulations is suggested by similarity between observations and simulations in the case of warm nights and extreme rainfall in some regions. For any specific extreme temperature index, minor differences appear in the spatial distribution of the changes across models in some regions, while substantial differences appear in regions in the interior of tropical and subtropical South America. The differences are in the relative magnitude of the trends. Consensus and significance are less strong when regional patterns are considered, with the exception of the La Plata Basin, where observed and simulated trends in warm nights and extreme rainfall are evident.     

A possible linkage in the interdecadal variability of rainfall over North China and the Sahel

1. Introduction The capital city of China, Beijing, and other largecities, such as Tianjin and Shijiazhuang are all locatedin North China (roughly the region 35?–40?N, 110?–125?E), a region with a very large population. In thepast several decades, the rainfall decreased remarkablyin North China. Nevertheless, the amount of waterdemanded by agriculture and industry and the popu-lation at large has increased. These two trends havemade the de?ciency of water resources in North Chinabeco…     

SST versus Climate Change Signals in West African Rainfall: 20th-Century Variations and Future Projections

Rainfall variability is a crucial factor in food production,water resource planning and ecosystems, especially in regions with scarce freshwaterresources. In West Africa rainfall has been subject to largedecadal and interdecadal variations during the 20th century. The most prominent feature is thereduction in rainfall amount throughout the second half of the century with somerecovery at the end. Among the conceivable mechanisms, which might inducesuch low-frequency variability in West African precipitation, this study isfocussed onsea surface temperature (SST) variations and increasing greenhouse gas (GHG)concentrations. A tool is presented to distinguish between both impacts bymeans of various climate model simulations, which are found to reproduce theobserved rainfall characteristics over West Africa reasonably well.Further, a multi-model approach is usedto evaluate the expected future greenhouse signal in West African rainfall with respect to natural variability and intermodel variations.It is found that observed SST fluctuations, forcing two different atmospheric climate models, are able to reproduce the main features ofobserved decadal rainfall anomalies in the southern part of West Africathroughout the second half of the 20th century. The seasonal response to varying SST isstrongest in summer when the region is undergoing intensive monsoondynamics. Whereas both atmospheric models simulate the observeddrought tendency,following the 1960s, there is some indication that the additional GHG forcing in one model inducessome significantly different rainfall anomalies in recent years, re-initiatingeven positive anomalies relative to the climatological mean which has alsobeen observed since the 1990s. However, thisresult is still subject to model uncertainty.Coupled climate model integrations with different climate change scenariosalsopredict that precipitation, particularly over the Guinea Coast and Sahelregion, will steadily increase into the 21st century. The model-comprehensive signal isstatistically significant with respect to natural variability and modeluncertainty, suggesting that the observed recovery of yearly rainfall overparts of West Africa might actually reflect the beginning impact of risinganthropogenic GHG. The physical mechanism, linking the radiative forcing tothe monsoonal rainfall, probably works via warming of the tropicalAtlantic Ocean.